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98 xj oxygen sensor

its an xj thing

NAXJA Forum User
Location
Wisconsin
my 98 4.0 has had a rough idle and low mpg since i bought it. i checked the 02 sensors with a obd2 scanner and looked at live data graphes to see what they were doing.

the pre cat 02 sensor seemed to be changing voltage quickly and consistently while the downstream sensor was pretty lazy.

i unplugged the upstream sensor to see how it would affect the drive-ability and the only thing that changed was the check engine light came on. absolutely no difference in running or driving.

would this indicate the upstream sensor is bad? i bought a new one just in case, but would like some input on what others think.
 
Unplugging the sensor isn't a good test. That scantool though should have told you how they were functioning.

If you bought a new one and the current ones has between 100k - 150k or more on them, I would replace it. Oxygen sensors have a finite lifespan, which varies a bit depending on engine efficiency.

They can get "lazy" before they set a code.
 
i dont have a good history on parts with this jeep. but the jeep does have 225k on it and i know they are supposed to be replaced as a maintenance item.

i realize unplugging the sensor is not a good test to see how it is functioning, but i would assume that it would run worse with it unplugged if the sensor were good.
 
How long had it been running when you unplugged it? For the first minute or two it's in open loop and ignoring the O2 sensor anyways, after that I would expect it to run very badly.

At 225k it doesn't owe you anything, but it sounds like it might be fine to me. I'd expect the downstream to be very lazy as it should be seeing mostly the same mixture at all times after the catalytic gets done with what's left over from the engine.
 
i drove it with the sensor unplugged for about 5-10 minutes without any change in running. and the engine was at operating temp when i unplugged it. only change was the check engine light came on
 
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